The present invention relates to a digital color copier for forming an image based on an image signal read by, for example, a scanner.
In conventional apparatuses such as facsimile machines and printers which output images, an image is compressed by using an image compression/decompression technique to utilize a transmission path and a memory effectively.
With convention compression techniques, the amount of compressed data has varied depending on the image to be compressed because such techniques have been essentially intended for compressing each image to a size as small as possible.
However, apparatuses for handling images include devices such as page memories in printers and copiers and video memories in displays for which it is important to treat all images as having the same size in consideration to the system configuration. Such apparatuses have relied upon compression techniques on a fixed length basis which always compress any image into a fixed size with a small working buffer size. Conventional techniques which satisfy such a requirement exists, for example, U.S. Pat. No. 5,220,440. This U.S.P. has disclosed the technique in that the quantization is performed corresponding to the gradation of the character, and the image data is quantized into the data of the consistent data format. However, because the technique disclosed in the U.S.P. discriminates the image not for the character and the photograph, it occasionally happens that the image can not obtain sufficient resolution and gradation for the character image area and the photograph image area respectively and simultaneously.
It is an object of the present invention to provide a method and apparatus for compressing an image in which required resolution and tone characteristics are maintained depending on the type of the image and in which compressed data can be obtained in a consistent data format with a constant compression ratio maintained.
It is another object of the present invention to provide a method for encoding an image in which encoding can be easily performed on a fixed length basis with a high compression ration and less deterioration of image quality and in which reverse variable length compression can be carried out even after fixed length compression, an image encoding apparatus and an apparatus for decoding encoded data using the same method.
It is still another object of the invention to provide an image encoding apparatus and an apparatus for decoding encoded data in which a high resolution image can be compressed at a high compression ration with less deterioration of image quality attributable to compression in a small scale circuit; the compression gives less influence on image quality even in an unstable output device; deterioration attributable to compression can be suppressed even in an apparatus having different color reproduction rates for the input and output signals; an apparatus for inputting and outputting image can prevent deterioration increasing output signal more than input signal; editing can be performed with a small circuit scale; and reverse variable length compression can be carried out even after fixed length compression.
It is still another object of the present invention to use such an image encoding apparatus and image decoding apparatus to provide an image forming apparatus in which memory resources can be effectively utilized and in which the efficiency of an image forming process can be improved.
According to a first embodiment of the invention, there is provided an image forming apparatus comprises means for storing image information; means for identifying types of images from the stored image information; means for determining resolution and the number of quantization steps in accordance with the result of identification; and means for quantizing the image information through an error diffusion process on at least one of the types of images based on the resolution and number of quantization steps determined by the determination means.
The present invention having the above-described structure allows the resolutions and numbers of quantization steps of, for example, a photographic image and a character image to be determined with priority given to tone characteristics for the photographic image and to resolution for the character image instead of compressing the image information at one compression ratio as in the prior art. The image information is quantized through an error diffusion process based on the resolutions and the numbers of quantization steps. This makes it possible to compress the image as a whole at a compression ratio of a consistent level while maintaining required tone characteristics for the photograph and required resolution for the character information. Further, since an error diffusion process is performed on at least the photographic image, high tone characteristics can be maintained in microscopic regions thereof. It is noted further that an image can be reproduced preferably as a whole even when the error diffusion process is carried out only in photographic regions and not in regions of characters and line arts.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.